This invention relates to high-performance, ultra-linear, i.e. distortion-free, audio amplifiers. The invention is particularly concerned with tube-type amplifiers in which the power stage includes a pair of vacuum tube amplifier devices connected in push-pull.
Vacuum tube audio amplifiers have remained attractive for high performance audio use, because of their extreme linearity and distortion-free characteristics. Class B amplifiers are employed for this because of their relatively high output power and efficiency. However, in push-pull operation, class B amplifiers do produce notch or crossover distortion when operated at low power levels.
Push-pull class B output stages draw grid current in operation. This grid current is supplied to it by a driver stage, which can be a triode device, operated in class A. This can be configured to drive a transformer having a low-resistance secondary coupled to the power or output tube grids. It is preferred to employ a push-pull driver stage formed of cathode-follower triodes. However, in any case the problem of crossover distortion remains.